Excimer lasers are currently used for the integrated circuit lithography. They provide a light for exposure of integrated circuits with the wavelength of 248 nm (KrF lasers) of 193 nm (ArF lasers). These wavelengths are in deep UV region and allow printing of smaller features as compared to the previous generation illumination sources based on I-line and G-line mercury lamps with the wavelengths of 365 nm and 435 nm. Laser based deep UV exposure tools such as steppers and scanners allow the exposure of circuits with critical dimensions of less than about 0.3 .mu.m.
As a result of industry transition to deep UV lithography the cost of light source as well as the overall exposure system has increased substantially. On the other hand, the productivity of these exposure tools, usually measured in number of wafers exposed per hour as well as the size of the wafers has also increased. As a result of all these changes, the cost of operation has increased which means that the cost of downtime has also increased.
The excimer laser is a sophisticated piece of equipment and is commercially available from suppliers such as Cymer, Inc. Even though the state of the art excimer lasers are normally very reliable pieces of equipment, they do break down occasionally. Moreover, they do require certain preventive maintenance to be performed on a relatively regular basis. For example, the working gas mixture is normally replaced every 100 hours. Certain individual components of the laser have limited life and therefore should be replaced periodically. For example, optical components, such as windows in the laser chamber and the output coupler should be replaced on a relatively regular basis. After somewhat larger number of pulses, which might be equivalent of several months or even years of laser operation, core modules of the laser should be changed as well. Such core modules include, for example, laser discharge chamber, optical-line narrowing module, power supply, pulse power module, etc. These operations maintenance operations typically require downtimes of a few hours to possibly a few days.
When the laser is down for whatever reason, the whole illumination system is down which might be very expensive, possibly up to many thousand dollars per hour.
Therefore, the object of the present invention is to provide a microlithography exposure system which avoids lithography system down time due to laser down time.